Resurgent cauldrons are defined as cauldrons (calderas) in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome. Seven of the best known resurgent cauldrons are: Valles, Toba, Creede, San Juan, Silverton, Lake City, and Timber Mountain. Geologic summaries of these and Long Valley, California, a probable resurgent caldera, are presented.Using the Valles caldera as a model, but augmented by information from other cauldrons, seven stages of volcanic, structural, sedimentary, and plutonic events are recognized in the development of resurgent cauldrons. They are: (I) Regional tumescence and generation of ring fractures; (II) Calderaforming eruptions; (III) Caldera collapse; (IV) Preresurgence volcanism and sedimentation; (V) Resurgent doming; (VI) Major ring-fracture volcanism; (VII) Terminal solfatara and hot-spring activity. These stages define the terminal cycle of resurgent cauldrons, which in the Valles caldera spanned more than 1 million years.The known and inferred occurrence of the seven stages in the eight cauldrons discussed, together with some time control in four cauldrons, indicates that resurgent doming is early in the postcollapse history; hence, it seems part of a pattern and not fortuitous. Doming of the cauldron block by magma pressure is preferred to doming by stock or laccolithic intrusion, although these processes may be subsidiary. Magma rise that produces doming may be explained in several ways, but the principal cause is not known. Nor is it known why some otherwise similar calderas do not have resurgent domes, although size and thickness of the cauldron block and the degree to which it was deformed during caldera collapse may be factors. All known resurgent 613 on May 30, 2015 memoirs.gsapubs.org Downloaded from 614 STUDIES IN VOLCANOLOGY structures are larger than 8 miles in diameter and are associated with silicic and, presumably, high-viscosity magmas.Genetically, resurgent cauldrons belong to a cauldron group in which subsidence of a central mass takes place along ring fractures and is related to eruption of voluminous ash flows, thereby differing from Kilauean-type calderas. It is proposed that typical Krakatoan-type calderas differ in that collapse is chaotic and ring fractures are not essential to their formation. Krakatoan calderas typically occur in the andesitic volcanoes of island arcs or the eugeosynclinal environment, and their sub-volcanic analogues are not known, whereas resurgent and related Glen Coe-type cauldrons are more common in cratonic or post-orogenic environments as are their sub-volcanic analogues -granitic ring complexes. Granitic ring complexes, such as Lirue, Sande, Ossipee, and Alnsj0, are probably the closest sub-volcanic analogues of resurgent calderas.The source areas of most of the ash-flow sheets of western United States and Mexico are yet to be found. It is suggested that many of them will prove to be resurgent structures.Present evidence suggests that ore deposits are more commonly associated with resur...

Long Valley caldera, a 17‐ by 32‐km elliptical depression on the east front of the Sierra Nevada, formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68‐0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende‐biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende‐biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self‐sealing of near‐surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated north‐west‐trending Sierra Nevada frontal faults that tap hot water at depth.

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Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcatfic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming •ruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8-10 km deep with smaller cupolas as shallow as 4-5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, [he record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1-0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8-6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000-3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity. along the Inyo/Mono Craters chain [Miller, 1985; Sieh, 1984]. These are the most recent eruptions within the chain of Quaternary volcanic centers that extends northward through eastern California from the Salton Trough in the south to the south end of the Cascade Range in northern California. With the exception of the active Caõcade volcanoes these are the most recent eruptions in the conterminous United States. Long Valley, Yellowstone• and Valles are the only silicic calderas in the conterminous United States to produce major, caldera-forming, ash flow eruptions in roughly the last 1 m.y.. The Long Valley region, which is on the boundary between the Sierra Nevada and Basin and Range provinces, illustrates